linux-kernel - Re: [PATCH] fs/resctrl: Slightly optimize cbm

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Message-ID: <aQjXLOHQevfQDhL0@e133380.arm.com>
Date: Mon, 3 Nov 2025 16:24:12 +0000
From: Dave Martin <Dave.Martin@....com>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc: Tony Luck <tony.luck@...el.com>,
	Reinette Chatre <reinette.chatre@...el.com>,
	James Morse <james.morse@....com>, Babu Moger <babu.moger@....com>,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] fs/resctrl: Slightly optimize cbm_validate()

Hi,

On Sat, Nov 01, 2025 at 02:40:58PM +0100, Christophe JAILLET wrote:
> Le 27/10/2025 à 12:43, Dave Martin a écrit :
> > Hi,
> > 
> > [Tony, I have a side question on min_cbm_bits -- see below.]
> > 
> > On Sun, Oct 26, 2025 at 08:39:52AM +0100, Christophe JAILLET wrote:
> > > 'first_bit' is known to be 1, so it can be skipped when searching for the
> > > next 0 bit. Doing so mimics bitmap_next_set_region() and can save a few
> > > cycles.
> > 
> > This seems reasonable, although:
> > 
> > Nit: missing statement of what the patch does.  (Your paragraph
> > describes only something that _could_ be done and gives rationale for
> > it.)
> 
> Will add it in v2.

Thanks

[...]

> > > For the records, on x86, the diff of the asm code is:
> > > --- fs/resctrl/ctrlmondata.s.old        2025-10-26 08:21:46.928920563 +0100
> > > +++ fs/resctrl/ctrlmondata.s    2025-10-26 08:21:40.864024143 +0100
> > > @@ -1603,11 +1603,12 @@
> > >          call    _find_first_bit
> > >   # ./include/linux/find.h:192:  return _find_next_zero_bit(addr, size, offset);
> > >          movq    %r12, %rsi
> > > -       leaq    48(%rsp), %rdi
> > > -       movq    %rax, %rdx
> > > +# fs/resctrl/ctrlmondata.c:133:        zero_bit = find_next_zero_bit(&val, cbm_len, first_bit + 1);
> > > +       leaq    1(%rax), %rdx
> > >   # ./include/linux/find.h:214:  return _find_first_bit(addr, size);
> > >          movq    %rax, 8(%rsp)
> > >   # ./include/linux/find.h:192:  return _find_next_zero_bit(addr, size, offset);
> > > +       leaq    48(%rsp), %rdi
> > 
> > (This is really only showing that the compiler works.  The real
> > question is whether the logic is still sound after this change to the
> > arguments of _find_first_bit()...)
> 
> Will remove in v2, if not useful.

It's harmless, but a bit of a distraction...

> > >          call    _find_next_zero_bit
> > >   # fs/resctrl/ctrlmondata.c:136:        if (!r->cache.arch_has_sparse_bitmasks &&
> > >          leaq    28(%rbx), %rdi
> > > ---
> > >   fs/resctrl/ctrlmondata.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/resctrl/ctrlmondata.c b/fs/resctrl/ctrlmondata.c
> > > index 0d0ef54fc4de..1ff479a2dbbc 100644
> > > --- a/fs/resctrl/ctrlmondata.c
> > > +++ b/fs/resctrl/ctrlmondata.c
> > > @@ -130,7 +130,7 @@ static bool cbm_validate(char *buf, u32 *data, struct rdt_resource *r)
> > >   	}
> > >   	first_bit = find_first_bit(&val, cbm_len);
> > > -	zero_bit = find_next_zero_bit(&val, cbm_len, first_bit);
> > > +	zero_bit = find_next_zero_bit(&val, cbm_len, first_bit + 1);
> > 
> > Does this definitely do the right thing if val was zero?
> 
> Yes, IMHO, it does.
> 
> If val is zero, first_bit will be assigned to cbm_len (see [1]).
> Then, find_next_zero_bit() will do the same because 'first_bit + 1' will
> overflow the size of the bitmap. (see [2] and [3])

Right, I think that works.

> The only case were we could have trouble would be to have 'first_bit + 1'
> overflow and be equal to 0. I don't think that such a case is possible.

I looks impossible to me: first_bit comes from
find_first_bit(..., cbm_len), so I don't think it can be greater than
cbm_len.

> > >   	/* Are non-contiguous bitmasks allowed? */
> > >   	if (!r->cache.arch_has_sparse_bitmasks &&
> > 
> > Also, what about the find_first_bit() below?
> 
> Should be updated as well.
> Will send a v2.

OK, sounds fair.

Cheers
---Dave

[...]

> [1]:
> https://elixir.bootlin.com/linux/v6.18-rc3/source/include/linux/find.h#L203
> [2]:
> https://elixir.bootlin.com/linux/v6.18-rc3/source/include/linux/find.h#L185
> [3]: https://elixir.bootlin.com/linux/v6.18-rc3/source/lib/find_bit.c#L55